Display screen or portion thereof with a graphical user interface



The sole FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing our new design.

The outermost broken line rectangle illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines and the dots connecting text features illustrate portions of the graphical user interface and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 